Automated Weather Observation Systems

The Automated Weather/Surface Observing System (AWOS/ASOS) is a suite of sensors which measure, collect and disseminate weather data to help meteorologists, pilots and flight dispatchers prepare and monitor weather forecasts, plan flight routes, and provide necessary information for correct takeoffs and landings. AWOS/ASOS installations provide a minute-to-minute update that is usually furnished to pilots by a VHF radio on a frequency between 118 and 136 MHz, by a telephone link, or through regular weather dissemination channels. AWOS/ASOS's are categorized as either Federal or Non-Federal. Federal AWOS/ASOS's were purchased and are currently maintained by the FAA. Non-Federal AWOS/ASOS's are purchased and maintained by state, local, and private organizations. The sensors measure weather parameters such as wind speed and direction, temperature and dew point, visibility, cloud heights and types, precipitation, and barometric pressure. AWOS/ASOS does not predict weather, but many send current information to weather offices where forecasts are produced using this information along with computer model outputs, satellite photos and radar images, to name a few.

Every hour on the hour, the AWOS data is made available to off-site users by those AWOS/ASOS's on Service A (long line telephone communication) or satellite uplink. Should conditions change rapidly, SPECIAL reports may also be made available. The aviation community, which is one of the largest users of environmental data, is the major user of the AWOS information.

AWOS/ASOS makes weather information available to pilots which was previously unavailable, except at airports with trained human weather observers. While the implementation of AWOS/ASOS has been helpful to pilots operating under Part 91 (most of us), it has been most important to those operators under Parts 135 and 121. Under those parts of the Federal Aviation Regulations, operators may not conduct instrument operations to airports without weather reporting, and may not even begin an instrument approach unless the reported weather is at or above minimums prior to reaching the final approach fix.

As a result, many airports which could not be reached by commercial operators are now accessible. More important, the availability of up-to-the minute weather information results in increased operational safety for all.


SuiteAWOS Station
The typical AWOS or ASOS suite is installed at a location on the airport surface that is presumed to be representative of the area. The installation here is at Sussex Airport, New Jersey. It is not installed near the runway threshold, but at many airports, the installation is near a runway. Placement of the equipment may have effects on the accuracy of observations, as will be discussed later.

Typical AWOS/ASOS Report

METAR KCDW 270953Z AUTO 18019G25KT 3SM BR BKN007 BKN011 19/18 A2999 RMK AO2 CIG 005V009 SLP159 T01890183 TS DSNT SE MOV E TSB00E37


METAR - Tells us it is an hourly meteorological report

KCDW - Tells us it was taken at Essex County airport (CDW) New Jersey

270953Z - Tells us the observation was taken on the 27th day of the month at 0953Z, 0553 local time (EDT)

AUTO - Tells us it is an automated report (no human input)

18019G25KT - Tells us the wind is from 180 magnetic at 19 Knots gusting to 25 Knots (tower winds)

3SM BR - Tells us the visibility is 3 Statute Miles in mist

BKN007 BKN011 - Tells us that the ceiling is at the broken layer of 700 feet AGL, and that there is a higher broken layer that is detectable at 1100 feet AGL

19/18 - Tells us that the temperature is 19 degrees Celsius and the dewpoint is 18 degrees Celsius

A2999 - Tells us the altimeter setting is 29.99 inches of mercury

RMK - This is the area where remarks and clarifying information is placed.

AO2 - Tells us that this is an automated weather observation by an AWOS/ASOS installation with a precipitation discriminator. Had it indicated A01 it would have meant that the installation is unable to discriminate the different types of precipitation.

CIG 005V009 - This tells us that over a period of time, the ceiling has been variable between 500 and 900 feet AGL.

SLP159 - Tells us that the Sea Level Barometric Pressure is 1015.9 Millibars. All reported altimeter settings and pressures are corrected to sea level and are not the actual pressure at the site.

T01890183 - Tells us that the EXACT temperature is 18.9 Celsius, and that the EXACT dewpoint is 18.3 Celsius.

TS DSNT SE MOV E TSB00E37 - A thunderstorm was detected in the distant southeast, and is moving east. The thunderstorm began at the hour and ended 37 minutes past the hour.


TSNO - Thunderstorm detection is not available

Determination of Wind Direction and Velocity

Wind direction and velocity is determined by a standard wind vane and anemometer installation on a pole near the AWOS/ASOS installation. This is the setup at Sussex, New Jersey.

To the left is the wind vane, and to the right is the anemometer. The rod sticking up in the middle is a lightning rod. The data developed is transmitted by wire to a data processing unit at or near the AWOS/ASOS installation.

AWOS/ASOS winds are treated as TOWER winds and are thus reported in degrees referenced to MAGNETIC North, and in the stated speed format.

Determination of Visibility

The time averaged prevailing visibility is determined through the use of a Xenon flash forward scatter system consisting of a transmitter and a receiver, Belfort Model 6220. Each unit is pointed slightly downward and offset so that the flash does not directly go from the transmitter to the receiver, but rather to a 9 inch spot in the atmosphere somewhere in between. This sensor unit is elevated above the surface and is located at Sussex Airport, New Jersey. The shot was taken facing Northwest; the transmitter is at the right and the receiver is at the left. The flash is visible to the observer. The wind instruments are visible in the background.

In practice, the transmitter unit sends a flash of light. The light reflects off particles in the air, and is sensed by the receiver unit. The amount of reflected light that is picked up by the receiver unit is sent to the computer and converted to Sensor Equivalent Visibility. The greater the number of reflective particles in the air, the greater the amount of light reflected, and the lower the Sensor Equivalent Visibility.

Determination of Precipitation and Obstructions to Visibility

The type of precipitation, if any, falling at an AWOS/ASOS installation is determined by a device known as a Precipitation Discriminator. It is actually called a Light Emitting Diode Weather Indicator (LEDWI).

The unit operates on the principle that different types of precipitation have differing values of reflectivity to light. The discriminator operates similarly to the unit which generates visibility values. In this case, however, the transmitter and receiver are pointed at each other. The data generated is sent to the computer which determines the type of precipitation which is falling, if any.

In the case where moisture is present at the site, another detector known as a Rosemount Sensor comes into play to determine whether it is freezing when it contacts objects on the ground. The unit operates on the principle of a tuning fork. The presence of a freezing substance is reflected by a change in the resonant frequency of the sensor, which is the small rod extending up from the cone at the top of the unit. A change in the resonant frequency within the parameters of the unit results in an indication of FZ as a modifier to indicate that whatever moisture is present is freezing upon contact with objects on the ground. In order to provide accurate indications of this very hazardous weather condition, the sensor is heated frequently to remove any frozen moisture which may have accumulated, so that the indication is timely and accurate. The little cone is the deicing element, and the large heat sink is just below the shaft.

Determination of Liquid Precipitation Equivalent Amounts

The amount of liquid precipitation is determined through the use of a tipping bucket made by Frise Engineering of Baltimore.

The blue skirt around the unit is a wind skirt, while the white cylinder in the center contains the measuring apparatus.

In practice, the precipitation enters the top of the cylinder through a funnel which directs it to a one of a pair of buckets which can hold .01 inch of liquid. As a bucket fills to capacity, it tips, empties, and places the other bucket in a position to fill. As the tipping occurs, a data pulse is sent to the computer to register .01 inch of precipitation.

Determination of Cloud Heights and Ceiling

This picture is of the ASOS CEILOMETER Located at Sussex Airport, New Jersey. Behind it you can see the freezing precipitation detector.

The ceilometer shoots a series of light pulses vertically and sends the received data to the computer. The ceiling algorithm converts the reading to a time averaged report of cloud heights and coverage.

Determination of Temperature and Dewpoint

This device is used to determine temperature and dewpoint, the values of which are very important in day-to-day aviation operations. Temperature is determined with an electronic thermometer. Unlike in the past, when a human observer determined the dewpoint through the use of a sling psychrometer (or still does), the AWOS/ASOS uses an electronic means to determine this value.

Very simply, a fan draws ambient air into the assembly where temperature and dewpoint are measured. Dewpoint is determined by drawing the air over a mirror which reflects light form a source to a sensor. The temperature of the mirror is cooled through a refrigeration process until moisture condenses on it, reducing the light reflected to the sensor. The temperature at which this occurs is measured and is the dewpoint.

As reflected in an automated METAR, the temperature and dewpoint are presented to the nearest degree, with the exact readings to a tenth of a degree presented near the end of the METAR format.

Determination Of Altimeter Setting And Sea Level Pressure

These pieces of information are developed by standard electronic means, and are adjusted to sea level. In most places in the US, the altimeter setting is presented in inches of mercury, and the Sea Level Pressure is presented in millibars. Rapid changes in pressure are reported.

Thunderstorm Detection

Thunderstorm detection is through standard sferics devices, similar to Stormscopes or Strikefinders. Based upon lightning strike data received, a report is made of the estimated location and movements of thunderstorms.

AWOA/ASOS Limitations

The use of AWOS/ASOS provides weather information where it might not be available, with some limitations. Most of the limitations of automated reporting revolve around the fact that the unit can only sample atmospheric conditions in a small area. While the sensors and computer algorithms used have been configured to provide maximum accuracy, the fact is that automated reporting can never be quite as good as the data from a human observer, particularly as regards ceiling and prevailing visibility.

An automated unit is only able to sample conditions in a small area, and it is unable to "see" beyond this area. For example, an automated unit may be enveloped in fog, but the runway environs may be clear. The unit cannot develop a figure for prevailing visibility as can a human observer, so you sill not see visibility data reflecting different visibilities in different directions.

Used with its limitations in mind, AWOS/ASOS provides valuable information and allows us to achieve a higher level of safety as compared to the days when we operated without weather reporting at the majority of airports.

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Roger F. Zaruba

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